A three spheres theorem for an elliptic equation of high order

In the sphere $Q\subset\mathbf R^n$ of unit radius we consider a uniformly elliptic equation of order $2m$ with simple complex characteristics and with coefficients in $C^{2m}$ satisfying a supplementary condition. Concerning continuous dependence on the initial conditions we prove a theorem that is analogous to Hadamard's three circle theorem in the theory of analytic functions.
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